Astronomical Images : Treatise title

Johann Dryander

Astronomical Images

<p style='text-align: justify;'>Johann Dryander (1500-1560) was an anatomist and physician as well as a mathematician and astronomer. He taught for a time at Paris and performed several dissections there, before being appointed Professor of Medicine and Mathematics at the University of Marburg in 1535. His astronomical publications, which include a work on astrolabes, as well as this treatise on astronomical rings, suggest that he was interested in the practical aspects of astronomy. The astronomical rings consist of three rings fashioned into one instrument. The instrument was invented by Gemma Frisius, who published a work on the rings in 1534, and so is sometimes known as Gemma's rings. The instrument could be used to tell the time and had the advantage that no further instruments were used to orientate it, since the meridian could be identified as the line of orientation at which the shadows of the instrument's rings align. It also had applications in surveying. Around two thirds of Dryander's work was devoted to describing the parts of his new version of the instrument and its markings, and there was a lengthy section on measuring the heights of objects. Following this section came a series of short treatises on different forms of the rings, listed on the <i>verso</i> of the title page, so that the whole work acted as a comprehensive guide to astronomical rings of all sorts. This page introduced the third treatise collected by Dryander. As with the second treatise, this appears to cover a ring dial, rather than the astronomical rings proper. This instrument could be used to measure altitudes and tell the time when a ray of light passed through a hole in the ring, onto the internal curved surface of the instrument. Unlike the astronomical rings, this instrument was not universal and could only be used at a given latitude. A table was supplied in the text of the treatise so that the instrument could be inscribed appropriately for the latitude of Marburg.</p>